1. Field of the Invention
The present invention relates to a glass antenna device for an automobile suitable for received signals in a long wave broadcast band (LW band)(150-280 kHz), a middle wave broadcast band (530-1630 kHz), a short wave broadcast band (SW band)(2.3-26.1 MHz), an FM broadcast band (76-90 MHz, (Japan)), an FM broadcast band (88-108 MHz (U.S.A.)), a TV-VHF band (90-108 MHz and 170-222 MHz) and a TV-UHF band (470-770 MHZ), which has a high signal receiving sensitivity and a noise suppressing property and which is rich in productivity.
2. Discussion of the Background
As a glass antenna device for an automobile which is capable of improving the signal receiving sensitivity by utilizing resonance, there has been proposed a glass antenna device for an automobile as shown in FIG. 7 (JP-Y-4-53070).
In this conventional example, a defogger 90 comprising heater strips 2 and bus bars 15a, 15b, 15c is provided in a rear window glass sheet 1 of an automobile, a choke coil 9 is connected between the bus bars 15a, 15b and a d.c. power source 10 for the defogger 90 wherein the impedance of the coke coil 9 is increased in a high frequency band region to thereby allow a direct current to flow from the d.c. power source 10 to the defogger 90, and to stop a current of high frequency band region such as a broadcast frequency band region, in which the defogger 90 is utilized as an antenna.
Further, a parallel resonance is generated by the stray capacitance to ground (hereinbelow, referred to simply as the stray capacitance) of the defogger 90 and a coil 71 in a middle wave broadcast band. Further, a received signal in the middle wave broadcast is passed by the coil 71, a capacitor 73 and a resistor 74. Reference numeral 11 designates a capacitor for cutting noises.
In the conventional example shown in FIG. 7, which employs the above-mentioned construction, an attempt has been made to improve the signal receiving sensitivity and to reduce noises.
However, in the conventional example, the stray capacitance of a cable which connects the defogger 90 to a receiver was an element to generate the parallel resonance. Further, the S/N ratio was poor because the parallel resonance frequency existed in a middle broadcast band, and the receiving sensitivity was insufficient because the resonance occurred in a single portion.
Further, when the defogger 90 was utilized as an antenna commonly used for a middle wave broadcast band and FM broadcast band and even when the shape of the defogger 90 was optimized for receiving middle wave broadcast signals, there were problems that the signal receiving sensitivity and the directivity for a FM broadcast were insufficient in a case of receiving middle wave broadcast signals.